There are many applications where it is necessary to control the level of a contained liquid between distinct upper and lower limits. Some examples are tankage in sewer works, water supply systems, and chemical and other processing industries. One particularly tough problem is removing oil as it collects in the well hole of a "stripper well". This last application will be discussed as an example of a liquid level controller throughout the specification.
The flow rate of many oil wells is far too slow to efficiently match with a continuous pumping speed to extract the oil as it seeps into the well. These wells, often called "stripper" wells, must be pumped intermittently to allow the oil to collect between pumpings. In such cases some provision must be made to turn the pumping device on when a quantity of oil has seeped into the bore and collected there to present a column of oil above the pumping mechanism of sufficient depth to economically warrant extraction. The pump should then run until most of the oil has been pumped to the surface, but must shut off before evacuating all the collected oil. It is very important to shut the pump off because running the pump dry will lead to rapid pump failure. For example, down-hole progressive cavity pumps will overheat and destroy their rubber seals and traditional walking beam pumps transmit devasting shocks throughout the entire pump when the down-hole pump mechanism begins to reciprocate at the surface of the oil. The compressible nature of air and the increased friction of running without the lubrication of a flow of oil are both serious contributing factors to pump failure if the pump is allowed to run dry.
The current common practice is to try to balance the pumping cycle with the flow of oil through a timing circuit. At selected intervals the pump runs for a predetermined period of time. The spacing between the intervals and the duration of each pumping period are adjusted to try to match the flow characteristics of the well. However any imbalance creates an error that is increased with each successive pumping cycle. Further, the flow characteristics of the well can change, and even if not, this imperfect approximation requires substantial man hours in calibrating and checking the system. A further disadvantage of the fixed timing cycles are that they favor collecting a relatively substantial column of oil before the pump is turned on. Extraction of large quantities of collected oil minimizes the proportion of pumping time during which the surface of the oil is nearest the pump mechanism. However, such a relatively tall column of oil has substantial weight and the pressure head developed inhibits the flow of oil from the reservoir into the well hole.
Thus it is clear that there is a present need to develop a detection system that will cut the pump on and off as a function of the actual oil level rather than as a function of where someone has anticipated the oil level should be at a given time. Such a system must quickly respond to changes of oil level and should be highly reliable in order to operate for extended periods without supervision. Further this liquid level controller must be able to control the pump even if the pump itself is distant from the pump mechanism at the oil level. A walking beam pump is an example of this kind of pump. A similar problem is presented by down-hole pumps such as a progressive cavity pump because the present control circuitry is on the surface. The present invention satisfies each of these needs and brings additional advantages to this application.